This invention relates to optical systems in general and more particularly to a three mirror folding system useful in combination with an annular reflective projection system to facilitate scanning an image and an object.
In copending application of H. S. Hemstreet et al., Ser. No. 339,860, filed Mar. 9, 1973 and assigned to the same assignee as the present invention, an optical projection and scanning apparatus is disclosed. The apparatus disclosed therein is a photographic device useful to affect the exposure to photoresist-coated semiconductor wafers in the manufacture of integrated circuits. In its operation, an arcuate area of an exposure-control mask is illuminated and the projection system images the illuminated portion of the mask onto the wafer. Synchronous motion of both the mask and wafer with respect to the illuminated region permits the entire wafer surface and corresponding mask surface to pass through the illuminated region thus resulting in imagery over the entire wafer. The apparatus uses an annular projection system of the type disclosed in U.S. Pat. No. 3,748,015 issued July 24, 1973. With nothing further, the employment of this projection system for the desired purpose would require movement of the image receiving surface, i.e., the wafer, across the imaged area in a direction opposite to the direction of relative movement of the projection mask or the like. Clearly, such is not desirable.
Thus it can be seen that, for this purpose, a simple projection apparatus enabling simultaneous movement of both the mask and wafer in a single direction is needed. In addition, it is desirable in such apparatus that the image projected be of the same orientation or symmetry as obtained in contact printing.